In 1997, highly pathogenic avian influenza H5N1 was transmitted from poultry to humans in Hong Kong, resulting in eighteen infected people and six deaths, and reemerged in 2003 causing two similar cases with one fatality. In 2003-2005, extensive outbreaks of H5N1 influenza occurred in nine Asian countries resulting in 19 human cases in Thailand, 91 in Vietnam, seven in Indonesia, and four in Cambodia, with a total of 62 reported deaths. Furthermore, H5N1 infections in family clusters have raised the possibility of human-to-human transmission. As human exposure to and infection with H5N1 viruses continues to increase, so, too, does the likelihood of the generation of an avian-human reassortment virus that may be transmitted efficiently within the global human population, which currently lacks H5N1 specific immunity. Such reassortment events between avian-human and swine-human influenza A viruses have been associated with the 1957 and 1968 influenza pandemics; the 1918 pandemic events remain unclear.
Concern over the potential for the generation of a pandemic H5 strain and its concomitant morbidity and mortality are spurring the search for effective agents against same.